<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
</head>
<style>
  #border-btn { 
    display: flex;
    align-items: center;
    justify-content: center;
    height: 100vh;
  }

  button {
    border: 0;
    border-radius: 10px;
    background: #2ec4b6;
    text-transform: uppercase;
    color: white;
    font-size: 16px;
    font-weight: bold;
    padding: 15px 30px;
    outline: none;
    position: relative;
    transition: border-radius 3s;
    -webkit-transition: border-radius 3s;
  }

  button:hover {
    border-bottom-right-radius: 50px;
    border-top-left-radius: 50px;
    border-bottom-left-radius: 10px;
    border-top-right-radius: 10px;
  }
</style>
<body>
  
  <div id="border-btn">
    <button>Hover me</button>
  </div>
</body>
</html>